Potential interactions between seismicity, fluid behaviours and aseismic deformation in the Western Sea of Marmara

The northern strand of the North Anatolian Fault (NAF) in the Sea of Marmara show a high seismic activity, including the recent Mw 6.2 earthquake of April 23, 2025, situated southwest of Istanbul. This fault zone is also characterized by different mechanical behaviours (i.e., locked vs creeping) and often associated with fluid evidences. In this study, we focus on the western part of the NAF in the Sea of Marmara, where aseismic deformation has often been reported to be at work. We use recordings from two piezometers, three ocean bottom seismometers (OBSs) from INGV, and three OBSs from KOERI, deployed around the Western High and the Tekirdağ Basin, to analyse the seismic activity between October 2013 and August 2014, and study potential links with pore pressure variations, a slow-slip event (SSE) that could have occurred during this period, and a Mw 4.6 earthquake that took place on November 27, 2013.This seismic network is completed by 11 land seismological stations to improve the microseismicity location accuracy.

In total, 2079 events were detected during the recording period, of which 409 events remained after double-difference relocation. We here identify a sequence of 21 highly-similar foreshocks during the week preceding the Mw 4.6 mainshock, aligned along sidewall faults in the Central Basin. This sequence coincides with the possible existence of a several months-long SSE propagating westwards, based on the interpretation of onshore geodetic data and offshore surface sediment pore pressure data. The foreshock occurrence, as well as the timing of the pore pressure variations measured within the fault valley, are compatible with the hypothesis that the modelled SSE impacted first the foreshock-mainshock sequence, and then fluid conditions within the NAF valley at the piezometer locations. Our results demonstrate that the combination of seafloor piezometry and seismology may prove very useful to study interactions between fluids and fault zone deformation, including preparatory phases of earthquakes.